Some what of a controlled leak. The PCV valve itself is more of the controlled restrictor than the 3/8 line.
With a vented catch can you would be pulling vacuum from the "vent" on the catch can and not on the crank case. Kinda defeats the purpose.
This chrome plated breather cap is ideal for race car vacuum balance systems, as well as OEM applications with positive crankcase ventilation. The breather features a 1'' O.D. neck and a 5/8'' O.D. breather tube, and an integral metal baffle/foam element combination. It fits all factory and aftermarket valve covers with a 1-1/4'' diameter hole. Grommet included.